Direct Synthesis of High-Valent Aryl–Cu(II) and Aryl–Cu(III) Compounds: Mechanistic Insight into Arene C–H Bond Metalation

Copper
and its salts are abundant, inexpensive, and eco-friendly
and have been used as the surrogates of noble metals to effect arene
C–H bond activation and transformations. Despite of the recent
significant progress of the study, syntheses of high-valent arylcopper­(II–III)
compounds are still very rare and mechanisms of copper­(II)-catalyzed
reactions remain elusive. With the use of azacalix[1]­arene[3]­pyridines
as a platform, a number of arylcopper­(II) compounds were synthesized
efficiently from the reaction of Cu­(ClO4)2 under
ambient conditions. The resulting aryl–Cu­(II) compounds, which
contain an unprecedented (substituted) phenyl–Cu­(II) σ-bond,
were stable under atmospheric conditions and can undergo facile oxidation
reaction by free copper­(II) ions or oxone to afford arylcopper­(III)
compounds in good yields. Both arylcopper­(II) and arylcopper­(III)
compounds were characterized unambiguously by means of XRD, XPS, and
NMR methods. Experimental evidence including reaction kinetics, LFER
and KIE, and theoretical calculations indicated that the Cu­(ClO4)2-mediated arene C–H bond activation proceeds
plausibly through an electrophilic aromatic metalation pathway. The
synthesis of high-valent arylcopper compounds and the reaction mechanism
reported here highlight the diversity and richness of organocopper
chemistry.